Light out-coupling arrangement and a touch sensitive system comprising the out-coupling arrangement

Abstract

The disclosure relates to a light out-coupling arrangement for a touch sensitive system comprising a light guide of a material with a refractive index n g and a top surface configured to be exposed to ambient light, and a bottom surface. The out-coupling arrangement comprises at least one detector arranged along the periphery of the light guide and configured to receive the light propagating in the light guide and an optical filter provided between the light guide and the at least one detector. The optical filter has a first side and a second side where the first side is arranged to face the at least one detector, and the second side is arranged to receive light from the light guide. The optical filter is made of a medium with a refractive index n f less than the refractive index n g of the light guide, such that ambient light incident onto the second side at an angle larger than a critical angle θ c from a normal of the second side will be reflected on the optical filter by total internal reflection and will not reach the at least one detector. The disclosure also relates to a touch sensitive system comprising a light out-coupling arrangement.

The invention claimed is: 
     
       1. A light out-coupling arrangement for a touch sensitive system, wherein the touch sensitive system comprises a light guide of a material with a refractive index n g  and a top surface configured to be exposed to ambient light, and a bottom surface, the touch sensitive system further has an in-coupling arrangement comprising at least one emitter arranged along the periphery of the light guide and is configured to emit light into the light guide whereby the light propagates in the light guide via total internal reflection, TIR,
 wherein the out-coupling arrangement comprises 
 at least one detector arranged along the periphery of the light guide and configured to receive the light propagating in the light guide; 
 an optical filter provided between the light guide and the at least one detector, wherein the optical filter has a first side and a second side where the first side is arranged to face the at least one detector, and the second side is arranged to receive light from the light guide, the optical filter is further made of a medium with a refractive index n f  less than the refractive index n g  of the light guide, such that ambient light incident onto the second side with an angle larger than a critical angle θ c  from a normal of the second side, will be reflected on the optical filter by total internal reflection and will not reach the at least one detector. 
 
     
     
       2. The out-coupling arrangement according to  claim 1 , wherein the optical filter is made of a medium with a refractive index n f  between 1 and 1.5. 
     
     
       3. The out-coupling arrangement according to  claim 1 , wherein the optical filter is arranged to reflect ambient light that is coupled out from the light guide through the lower surface of the light guide. 
     
     
       4. The out-coupling arrangement according to  claim 1 , wherein the optical filter is arranged to reflect ambient light that is coupled out from the light guide through an edge of the light guide. 
     
     
       5. The out-coupling arrangement according to  claim 1 , wherein the second side of the optical filter is oriented in relation to the light guide such that ambient light incident on the top surface of the light guide is incident on the second side with an angle larger than the critical angle θ c  from a normal of the second side and thus reflected by total internal reflection. 
     
     
       6. The out-coupling arrangement according to  claim 5 , wherein the second side of the optical filter is oriented with an angle θ 4  to a normal of the top surface of the light guide such that the second side faces away from the light guide. 
     
     
       7. The out-coupling arrangement according to  claim 5 , wherein the second side of the optical filter is oriented in a direction substantially perpendicular to the top surface of the light guide. 
     
     
       8. The out-coupling arrangement according to  claim 1 , wherein the optical filter has a width between the first side and the second side of at least 1 μm. 
     
     
       9. The out-coupling arrangement according to  claim 1 , wherein the optical filter is a tape. 
     
     
       10. The out-coupling arrangement according to  claim 1 , wherein the optical filter is a coating. 
     
     
       11. The out-coupling arrangement according to  claim 1 , wherein the medium of the optical filter is air. 
     
     
       12. The out-coupling arrangement according to  claim 1 , wherein the medium of the optical filter is a fluid. 
     
     
       13. A touch sensitive system comprising
 a light guide having a top surface and a bottom surface; 
 an in-coupling arrangement comprising at least one emitter arranged along the periphery of the light guide and configured to emit light into the light guide whereby the light propagates in the light guide via Total Internal Reflection, TIR, wherein the touch sensitive system further comprises an out-coupling arrangement according to  claim 1 .